This invention relates to a method for preparing reticulated foam structures of a crosslinkable thermosetting polymer.
It is well known that foam or cellular polymeric structures have many uses, such as the material of construction for various containers, and as packaging and insulating materials. Foamed polymers may also be used as the materials for wicks or applicators in applying a metered amount of a lubricant or other fluids.
A variety of methods for preparing foamed polymeric structures are known in the art. A number of such methods are disclosed in U.S. Pat. No. 3,753,932 to Jenkins. The invention of that patent relates to a process for making open-celled polymeric structures which involves the steps of dissolving the polymer in a chlorofluorocarbon solvent to form a polymer solution, cooling the solution to form a solidified solution, and removing the solvent from the solidified solution at a temperature below the solidification temperature of the solvent. If water is added to the polymer solution of Jenkins to produce a liquid emulsion, then the emulsion is cooled to a temperature above the solidification temperature of water (0.degree. C.) but below the solidification temperature of the solvent. The water is then removed from the solidified emulsion before the removal of the solvent. The method of Jenkins produces open-celled polymeric structures, but it requires the use of a chlorofluorocarbon solvent and the preparation of a polymeric solution at elevated temperatures, for example by refluxing. Moreover, Jenkins removes the solvent by sublimation from the solidified solution to leave the polymeric structure in cellular form. Accordingly, there is a need for an improved and economical method for making reticulated foam structure of a crosslinkable thermosetting polymer.
In Gros U.S. Pat. No. 3,868,337, there is disclosed a process for preparing a closed-cell foam from a vinylidene fluoride/hexafluoropropylene copolymer by vulcanizing the copolymer in the presence of a blowing agent and a vulcanization agent. The vulcanization in the process of Gros is carried out at elevated temperature and pressure. As the vulcanizing agent, Gros discloses the use of the oxides and hydroxides of lead and calcium, or the oxides and hydroxides of zinc and magnesium in combination with at least one lead or zinc salt.
In Schmidt U.S. Pat. No. 3,505,435, there is disclosed a method for producing a partially foamed thermoplastic container, which includes the steps of injection molding of a foamable preform and heating selected portions of the preform to form a partially foamed article. A cooling step to prevent further foaming follows the heating step. The patent states that at the point of end use, the foaming of the article may be completed by heating.
Finally, in Cellular Viton, Haaf et al, DuPont "Viton" Bulletin No. 27, 1968, TS1925 D95v, methods for preparing open-cell and closed-cell Viton sponges are disclosed. The method for making open-cell Viton sponge includes the steps of mixing the Viton formulation in a cold laboratory mill and forming the mixture into a sheet, curing the sheet under light pressure and elevated temperatures, and post-curing at a higher elevated temperature.